Behind the Mask

Boris Karloff

as Jim Henderson

Edward Van Sloan

as Dr. August Steiner / Dr. Alec Munsell / Mr. X

Constance Cummings

as Julie Arnold

Jack Holt

as Jack Hart aka Quinn

Willard Robertson

as Capt. E.J. Hawkes

Claude King

as Arnold

Jessie Arnold

as Eastland Hospital Nurse

Thomas E. Jackson

as Agent Burke

Martha Mattox

as Hotel Ansonia Maid

Harry Tenbrook

as Man in Black / Henchman in Boat